I use a .330 ball, as that was what my Uberti factory literature recommended. I know their bore and chamber specs have changed over the years, in my gun the .320 ball will move in the cylinder, as it is a loose fit. I also get no seal with balls under .330, and the performance suffers dramatically as a result. Over a chronograph, a .320 ball will go about 400 fps and bounce off a sheet of 3/8" wafer board. A .330 ball, using the same powder charge will get 950 fps and zip through a couple of 2x4's no problem. For those who think the .31 pocket gun is a pop-gun, watch this video: